1. Field of the Invention
This invention relates generally to the field of semiconductor circuit design, and more particularly to the design of improved power regulators.
2. Description of the Related Art
Many electronic power supplies feature voltage regulators, or regulator circuits, designed to automatically maintain a constant output voltage level to effectively provide a steady voltage to the electronic circuit to which power is being supplied, typically referred to as the load. More particularly, the object of a voltage regulator circuit is to maintain a steady output voltage regardless of current drawn by the load. Most present day voltage regulators operate by comparing the actual output voltage to a fixed—typically internal—reference voltage. The difference between the actual output voltage and reference voltage is amplified, and used for controlling a regulation element, to form a negative feedback servo control loop. The regulation element is typically configured to produce a higher voltage when the output voltage is too low, and in case of some regulators, to produce a lower voltage when the output voltage is too high. In many cases, the regulation element may be configured to simply stop sourcing current, and depend on the current drawn by the driven load to pull down the regulator output voltage. The control loop has to be carefully designed to produce the desired tradeoff between stability and speed of response.
The operation of power supplies is typically affected by variations on the input voltage (or power supply) line that provides the voltage based on which the regulated output voltage is generated. Any signal or noise (including transients, which may reach very high levels relative to the level of the desired output voltage) on the supply line may couple into, and may be amplified by the active circuitry, thereby degrading the performance of the power supply. Therefore, in addition to design considerations related to stability and speed of response, power supplies are also typically designed to achieve a desired power supply rejection ratio (PSRR), which is indicative of the amount of noise (on the supply line) that the power regulator is capable of rejecting. Various systems may specify different power supply rejection requirements.
Another important measure of the effectiveness of a voltage regulator circuit is its ability to quickly stabilize when responding to a demand for high current. For example, when the demand for current to be supplied by the voltage regulator suddenly changes, an ideal voltage regulator should be able to meet the demand for increased current while maintaining its desired output voltage Vout. However, this may not always be practical for a given voltage regulator circuit and a given load. For example, in many cases an external pass-device, typically a pass-transistor is used to ensure sufficient load current for high-voltage applications. As the load current quickly rises from no current to maximum load current, the voltage regulator may become unstable. Many present day implementations use a large internal pass transistor, and/or large current load at the output of the regulator to help stabilize the voltage regulator. However, system requirements oftentimes prevent the use of these devices, and other solutions might be preferable, or even required.
Many other problems and disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.